1. Field of the Invention
This invention pertains in general to ion conductors and to processes for the synthesis of ion conducting solid electrolytes. In particular, the invention relates to the use of nanoscale powders for the preparation of nanostructured oxygen ion conducting electrolytes.
2. Description of the Prior Art
Solid electrolytes are materials through which ion species can migrate with low energy barriers. Table 1 outlines some examples of ion-conducting structures, representative materials, and the ions conducted. These materials are of critical commercial importance to electrochemical devices, components and processes. Illustrative applications include sensors, batteries, fuel cells, ion pumps, membrane reactors, catalysis, and metallurgy.
As a specific example, stabilized zirconia is a known conductor of oxygen ions. Accordingly, its properties are utilized in various fields of technology, such as in oxygen sensors for fuel-air ratio optimization of automobiles and furnaces, in oxygen pumps for solid state oxygen separation, in solid-oxide fuel cells for noiseless and clean power generation from chemical energy, and in catalytic membrane reactors.
The oxygen-ion conduction properties of stabilized zirconia used in a typical oxygen sensor are well understood based on electrochemical-cell theory. When placed between two compartments containing a reference gas and an analyte oxygen gas at different partial pressures, stabilized zirconia functions both as a partition between the two compartments and as an electrochemical-cell electrolyte. Under ideal conditions, the open-circuit emf (E0) of the cell is given by the known Nernst equation:                                           E            0                    =                                    RT                              4                ⁢                F                                      ⁢                          ln              ⁡                              (                                                      PO                    2                    Ref                                                        PO                    2                                                  )                                                    ,                            (        1        )            
where T is the absolute temperature of the cell; PO2Ref, and PO2 are the partial pressures of oxygen in the reference and analyte compartments, respectively; R is the universal gas constant; and F is Faraday""s number.
According to this equation, any difference in partial pressure of the oxygen across the two faces of the oxygen-conducting electrolyte generates an electromotive force that depends on the temperature and partial-pressure ratio of the oxygen in the two compartments of the system. In order to generate Nernstian response in sufficiently short times, the temperature of stabilized ZrO2 needs to be high (above 700xc2x0 C.), which results in relatively high power requirements and in increased equipment mass and size, need for insulation, and attendant sealing problems. These considerations often produce unsatisfactory performance or affect the commercial viability of products based on stabilized ZrO2 technology.
The inherent reasons for the high-temperature requirement and the corresponding performance problems of present-day oxygen ion conducting electrolyte based devices can be traced to the reaction mechanism of the cell and the microstructure of the sites where the reaction occurs. Referring to FIG. 1, a schematic drawing of a ZrO2 sensor cell 10 is illustrated, where the stabilized zirconia is modeled as a solid electrolyte membrane 12 between a first compartment 14, containing a reference oxygen atmosphere at a predetermined partial pressure PO2Ref, and another compartment 16 containing an analyte gas with oxygen at a different partial pressure PO2. The two sides of the stabilized zirconia non-porous solid electrolyte 12 are coupled through an external circuit connecting an anode 18 and a cathode 20 made of porous metal, such as silver. The anode 18 is the cell electrode at which chemical oxidation occurs and the electrons released by the oxidation reaction flow from it through the external circuit to the cathode. The cathode 20 is the cell electrode at which chemical reduction occurs. The cell electrolyte 12 completes the electrical circuit of the system by allowing a flow of negative ions O2xe2x88x92 between the two electrodes. A voltmeter 22 is provided to measure the emf created by the redox reactions occurring at the interfaces of the electrolyte with the two oxygen atmospheres.
Thus, the key redox reaction of the cell occurs at the points where the metal electrode, the electrolyte and the gas meet (illustrated in the inset of FIG. 1 as the xe2x80x9ctriple pointxe2x80x9d 24 ). At each such site on the surface of the electrolyte 12, the redox reaction is as follows:
O2.(gas)+4exe2x88x92xe2x86x922O2xe2x88x92.xe2x80x83xe2x80x83(2)
Since the reaction and the electrochemical performance of the sensor depend on the redox kinetics, the cell""s performance is a strong function of the concentration of triple points. In other words, an electrode/electrolyte/electrode cell with as many triple points as possible is highly desirable [see Madou, Marc and M. Morrison, Chemical Sensing with Solid State Devices, Academic Press, Boston (1989)]. In the case of an oxygen cell with a ZrO2 solid membrane and silver electrodes, this requirement corresponds to maximizing the triple points on each side of the PO2.Agxe2x80x2/ZrO2/Agxe2x80x3PO2Ref system.
Another cause of poor performance of oxygen-sensor cells can be explained with the help of complex-impedance analysis. Referring to FIGS. 2a and 2b, a complex impedance diagram for a ZrO2 sensor is shown, where the impedances of the bulk, grain boundary and electrode are illustrated in series to reflect their contribution to the ionic conduction at each triple point. It has been shown that the conductive performance of electrolytes at temperatures below 500xc2x0 C. is controlled by the grain boundary contribution to the overall impedance. Thus, for significant improvements of the conductivity at low temperatures, it is necessary to significantly minimize the grain-boundary (interface) resistance.
In summary, oxygen ion conducting devices based on stabilized-zirconia electrolyte have two problems that can be traced to material limitations. First, the electrolytes have high impedance; second, the concentration of triple points is relatively low. These problems are common to solid oxygen-conducting electrolytes in particular and solid electrolytes in general, and any improvement in these material characteristics would constitute a significant technological step forward. The present invention provides a novel approach that greatly improves these aspects of ion conducting solid electrolytes.
One of the objectives of this invention is to enhance the ion-conductivity of solid electrolytes by preparing nanostructured solid electrolytes.
Another objective is to reduce the electrolyte thickness with the use of nanostructured precursors of solid electrolytes.
A further objective is to enhance the concentration of triple points in the ion conducting devices by using nanostructured precursors and materials.
Yet another objective of the invention is to utilize the unique properties of size confinement in solid electrolyte and electrode grains when the domain is confined to less than 100 nanometers.
Another objective of this invention is an oxygen-conducting electrolyte material with low-impedance oxygen conducting characteristics.
Another objective of the invention is an oxygen ion conducting device with a very high density of triple points.
Another goal is a process and materials that reduce the cost of manufacture of products that incorporate oxygen-ion conductors.
Yet another goal is a process and materials that reduce the cost of operation of products that incorporate oxygen-ion conductors.
Finally, another goal is a process that can be readily incorporated with conventional methods for manufacturing products containing ion-conducting electrolytes.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.